The invention relates to a low-pressure mercury vapor discharge lamp having, around an electric stabilization ballast which is necessary for the operation of the lamp, a discharge vessel which is closed in a vacuum-tight manner and which encloses electrodes between which a discharge is present during operation of the lamp. The discharge vessel contains mercury and a rare gas and is shaped and dimensioned such that the discharge path is curved in one or more places. Such a lamp is disclosed in U.S. Pat. No. 3,899,712.
This United States Patent discloses compact cylindrical low-pressure mercury vapor discharge lamps which can be placed in luminaires having holders for incandescent lamps for general lighting purposes. In the above-mentioned lamps the centrally positioned stabilization ballast is surrounded by the discharge vessel, which consists of two slightly tapered glass cylinders which bear against each other, a helical groove having been provided to form the discharge path in at least one of the cylinder walls.
A centrally positioned stabilization ballast makes it possible to reduce the dimension of the lamp to a minimum and to shape the lamp in such manner that it resembles an incandescent lamp. In the known lamp the stabilization ballast (consisting of, for example, a plurality of turns of insulated copper wire around a laminated iron core) is surrounded in such manner by the wall of the cylindrical discharge vessel that there is only a relatively small aperture at the top of the lamp. As a result thereof, because of a relatively poor ventilation, the temperature of the ballast can increase during operation of the lamp to an unwanted high value. Too high an operating temperature of the ballast results in deterioration of the insulation of the copper wire and in a reduced efficiency of the ballast. Added to this is the fact that the temperature in the discharge vessel increases during operation of the lamp to such a high value owing to heat radiation by the ballast that the optimum mercury vapor pressure for a highest possible conversion efficiency of electric power applied to the lamp into ultraviolet radiation is exceeded. This causes the luminous flux and the efficiency of the lamp to decrease.